1. Field of the Invention
This invention relates to processes for the manufacture of ethanol by continuous fermentation.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. The substitution of ethanol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of raw materials as possible so as to maximize the amount of product produced per unit of carbohydrate feed and thereby enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to optimizing the manufacture of ethanol from biomass and consequently, little effort has been made to eliminate or minimize the small but significant raw material losses which take place in each of the discrete operations involved in the manufacture of ethanol from vegetative sources.
Processes for the continuous fermentation of sugars to provide alcohol are well known (viz., U.S. Pat. Nos. 2,155,134; 2,371,208; 2,967,107; 3,015,612; 3,078,166; 3,093,548; 3,177,005; 3,201,328; 3,207,605; 3,207,606; 3,219,319; 3,234,026; 3,413,124; 3,528,889; 3,575,813; 3,591,454; 3,705,841; 3,737,323; and 3,940,492; "Process Design and Economic Studies of Alternative Fermentation Methods for the Production of Ethanol", Cysewski, et al. Biotechnology and Bioengineering, Vol. XX, pp. 1421-1444 (1978)). In a typical continuous fermentation process, a stream of sterile sugar liquor and a quantity of yeast cells are introduced into the first of a battery of fermentation vessels wherein initial fermentation takes place, generally under conditions favoring rapid cell growth. The partial fermentate admixed with yeast cells is continuously withdrawn from the first fermentation vessels wherein fermentation is carried out under conditions favoring the rapid conversion of sugar to ethanol. The yeast in the last fermentation vessel can be recovered by suitable means, e.g., centrifugation or settlement, and recycled. In the various known processes for obtaining fermentable sugar from starch by hydrolysis of the latter, particularly those which employ acid as the hydrolyzing agent, some of the product fermentable sugar will undergo chemical modification to provide oligomers, for example, dimers and/or trimers, which are not readily converted to ethanol using yeasts commonly encountered in the brewing industry. The oligomers which have resisted conversion to ethanol and are therefore present in the product "beer" stream at the conclusion of fermentation are recovered in the distillation bottoms during the process of concentrating the ethanol. Up until now, it has been necessary to re-hydrolyze the recovered oligomers to fermentable sugar and recycle the sugar re-hydrolysate to fermentation when maximum total utilization of raw material for the production of ethanol is desired. However, the additional capital investment needed to provide a plant having the capability to recover, rehydrolyze and recycle the oligomers, and the increased operational complexity and consumption of energy which this capability necessarily entails, have to date militated against the general adoption of the foregoing procedures. Thus, an otherwise valuable raw material for ethanol production, saccharide oligomer, is either being routinely discarded or diverted to uses other than ethanol production.